1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier or printer using an electrophotographic process, and a charging device which is preferably used for the image forming apparatus.
2. Related Background Art
In an image forming apparatus using an electrophotographic process, a photosensitive member is charged uniformly by charging means prior to an image exposure process.
In recent years, as a charging system of the charging means, a charge injection system has been contrived in which a discharge phenomenon is not substantially involved.
The charge injection system is a system in which a surface of a photosensitive member is charged as charges are injected directly into the photosensitive member from a contact charging member. In the charge injection system, a medium-resistance contact charging member comes into contact with the surface of the photosensitive member to directly inject charges into the surface of the photosensitive member not through a discharge phenomenon. In other words, the discharge is not generally used, and the charge injection is directly performed on the photosensitive member surface. Since the discharge phenomenon is not used, a voltage required for the charging is only that for a desired photosensitive member surface potential, and further, ozone is not generated. Thus, this is an ozone-less low-power charging system. In addition, in order to directly inject charges, a surface potential is equivalent to an applied voltage, and a charge start voltage Vth does not appear. Consequently, even in the case in which a DC voltage is applied, stable charging which is not affected by variation of environment such as humidity can be realized.
On the other hand, a probability of contact between the charging member and the surface of the photosensitive member influences charging ability due to a characteristic that charges are injected only into an area where the charging member comes into contact with the surface of the photosensitive member. In the case in which the probability of contact is insufficient and a large portion of the surface is not charged, charging ends before the photosensitive member surface potential reaches the voltage applied to the charging member.
In a magnetic brush charging device, or a contact charging device using nonmagnetic conducting particles described, for example, in JP 10-307454 A to 10-307459 A, a high probability of contact between a contact charging member and a surface of a photosensitive member is obtained uniformly, and a direct charge injection mechanism is predominantly employed.
In general, the magnetic brush charging device magnetically constrains conducting magnetic particles on a surface of a sleeve containing a magnet roller and rotates the sleeve, thereby performing charging while increasing a probability of contact between the sleeve and the surface of the photosensitive member.
The contact charging device using nonmagnetic conducting particles deposits conductive particulates on a sponge roller formed of a conductive sponge or the like and causes particles to intervene between the sponge roller and the surface of the photosensitive member, thereby making it possible to reduce a frictional resistance between the sponge roller and the surface of the photosensitive member and provide a difference of velocities between the sponge roller and the photosensitive member. Thus, a probability of contact between the sponge roller and the photosensitive member is improved through the difference of velocities and the intervention of the particles.
In this way, in the charge injection system, in order to eliminate contact failure between conducting particles and a member to be charged to obtain a sufficient chance of contact, it is preferable to bring the conducting particles and the member to be charged into contact with each other to prevent generation of a gap.
However, in the case in which the conducting particles are used, the conducting particles may deposit on the member to be charged and fall from the charging device.
In addition, in order to prevent charge injection ability from becoming insufficient due to speed-up of operations in the image forming apparatus, it is also considered to perform preliminary charging with a preliminary charger prior to charging with an injection charger.
If the preliminary charging is performed in this way, the fall of conducting particles becomes conspicuous, the conducting particles become insufficient, and charging failure may occur due to insufficient chance of contact.